io 
Dr. Fordyce's Account 
GGGG slides, in the same manner as in that part of a transit 
instrument which renders one end of the axis higher or lower. 
This second frame is brought higher or lower by the screw 
HH. The rod IB, fig. 1, is a brass tube, represented by II. 
There is a semicylindric cavity in the frame GGGG of the 
same diameter with the brass tube ; a piece is applied over 
this frame, which is likewise a semicircular cavity, forming 
with the other cavity a whole cylinder, equal in diameter to 
the tube I I. This piece is attached to the frame GGGG by 
the screws L L ; when these are screwed in tight, they occa- 
sion the tube I I to be embraced perfectly by the piece KK, 
and the semicylindric cavity in the frame G, so that the 
screw HH could not act if the tube I I was fixed ; but if the 
screws L L are loosened, then they would leave the frame 
GGGG capable of being heightened or lowered by the screw 
HH. Now, if I I represent the rod IB, fig. 1, then if the 
centre of the screws L L be in an horizontal line, that line 
will cut the point I in an horizontal direction. The centres 
of these screws, therefore, may be considered as the point I in 
perpendicular height ; wherefore if the point B, fig. 1, should 
become a fixed point, the point I may be lowered or heightened 
by bringing up or carrying down the centres of the screws L L 
by the screw HH. 
If it be wished, therefore, by this apparatus to raise or de- 
press the point I, fig. 1, it is necessary during the time of al- 
tering its height by the screw HH, that B, fig. 1, should be- 
come a fixed point ; but that at all other times it should be a 
part perfectly free in its motion upwards or downwards. 
In order to fix the point B fig. i, in fig. 3 AAAA represents 
the frame of the clock, BB another frame fixed to this frame, and 
